The gamma-tubulin complex protein Alp4 provides a link between the metaphase checkpoint and cytokinesis in fission yeast

Specific detection of fission yeast primary septum reveals septum and cleavage furrow ingression during early anaphase independent of mitosis completion

It is widely accepted in eukaryotes that the cleavage furrow only initiates after mitosis completion. In fission yeast, cytokinesis requires the synthesis of a septum tightly coupled to cleavage furrow ingression. The current cytokinesis model establishes that simultaneous septation and furrow ingression only initiate after spindle breakage and mitosis exit. Thus, this model considers that although Cdk1 is inactivated at early-anaphase, septation onset requires the long elapsed time until mitosis completion and full activation of the Hippo-like SIN pathway. Here, we studied the precise timing of septation onset regarding mitosis by exploiting both the septum-specific detection with the fluorochrome calcofluor and the high-resolution electron microscopy during anaphase and telophase. Contrarily to the existing model, we found that both septum and cleavage furrow start to ingress at early anaphase B, long before spindle breakage, with a slow ingression rate during anaphase B, and greatly increasing after telophase onset. This shows that mitosis and cleavage furrow ingression are not concatenated but simultaneous events in fission yeast. We found that the timing of septation during early anaphase correlates with the cell size and is regulated by the corresponding levels of SIN Etd1 and Rho1. Cdk1 inactivation was directly required for timely septation in early anaphase. Strikingly the reduced SIN activity present after Cdk1 loss was enough to trigger septation by immediately inducing the medial recruitment of the SIN kinase complex Sid2-Mob1. On the other hand, septation onset did not depend on the SIN asymmetry establishment, which is considered a hallmark for SIN activation. These results recalibrate the timing of key cytokinetic events in fission yeast; and unveil a size-dependent control mechanism that synchronizes simultaneous nuclei separation with septum and cleavage furrow ingression to safeguard the proper chromosome segregation during cell division.

Fission yeast cytokinesis requires the invagination of the equatorial plasma membrane (cleavage furrow ingression) coupled to the synthesis of a special wall structure named septum (septation). Despite Cdk1 kinase is inactivated in early anaphase, it is believed that cleavage furrow ingression and septation onset require anaphase progression and mitosis completion, only initiating after the complete activation of the Hippo-like septation initiation network (SIN) after telophase onset. Here, we studied the precise timing of septation start with respect to mitosis through specific septum-staining and electron microscopy. We found that septum and cleavage furrow ingression initiate in early anaphase, showing first a slow ingression rate during anaphase B, and increasing to a fast ingression rate after telophase onset. Thus, mitosis and cleavage furrow ingression are not concatenated but simultaneous events in fission yeast. The timing of septation correlated with cell size and depended on the level of cytoplasmic activators like SIN Etd1 and Rho1. We further analyzed the mitotic mechanisms that control the septation onset during early anaphase. Cdk1 directly regulated the timing of septation onset during early anaphase, and the low SIN activity present after Cdk1 inactivation was enough to trigger septation. Globally, these results recalibrate the timing of the main cytokinetic events of fission yeast and reveal a size-dependent control mechanism that synchronizes simultaneous nuclei separation with septum and cleavage furrow ingression.

γ-Tubulin complexes in microtubule nucleation and beyond

Tremendous progress has been made in understanding the functions of γ-tubulin and, in particular, its role in microtubule nucleation since the publication of its discovery in 1989. The structure of γ-tubulin has been determined, and the components of γ-tubulin complexes have been identified. Significant progress in understanding the structure of the γ-tubulin ring complex and its components has led to a persuasive model for how these complexes nucleate microtubule assembly. At the same time, data have accumulated that γ-tubulin has important but less well understood functions that are not simply a consequence of its function in microtubule nucleation. These include roles in the regulation of plus-end microtubule dynamics, gene regulation, and mitotic and cell cycle regulation. Finally, evidence is emerging that γ-tubulin mutations or alterations of γ-tubulin expression play an important role in certain types of cancer and in other diseases.

Mzt1/Tam4, a fission yeast MOZART1 homologue, is an essential component of the γ-tubulin complex and directly interacts with GCP3(Alp6)

In humans, MOZART1 plays an essential role in mitotic spindle formation as a component of the γ-tubulin ring complex. We report that the fission yeast homologue of MOZART1, Mzt1/Tam4, is located at microtubule-organizing centers (MTOCs) and coimmunoprecipitates with γ-tubulin Gtb1 from cell extracts. We show that mzt1/tam4 is an essential gene in fission yeast, encoding a 64-amino acid peptide, depletion of which leads to aberrant microtubule structure, including malformed mitotic spindles and impaired interphase microtubule array. Mzt1/Tam4 depletion also causes cytokinesis defects, suggesting a role of the γ-tubulin complex in the regulation of cytokinesis. Yeast two-hybrid analysis shows that Mzt1/Tam4 forms a complex with Alp6, a fission yeast homologue of γ-tubulin complex protein 3 (GCP3). Biophysical methods demonstrate that there is a direct interaction between recombinant Mzt1/Tam4 and the N-terminal region of GCP3(Alp6). Together our results suggest that Mzt1/Tam4 contributes to the MTOC function through regulation of GCP3(Alp6).

Fission yeast MOZART1/Mzt1 is an essential γ-tubulin complex component required for complex recruitment to the microtubule organizing center, but not its assembly

γ-Tubulin plays a universal role in microtubule nucleation from microtubule organizing centers (MTOCs) such as the animal centrosome and fungal spindle pole body (SPB). γ-Tubulin functions as a multiprotein complex called the γ-tubulin complex (γ-TuC), consisting of GCP1-6 (GCP1 is γ-tubulin). In fungi and flies, it has been shown that GCP1-3 are core components, as they are indispensable for γ-TuC complex assembly and cell division, whereas the other three GCPs are not. Recently a novel conserved component, MOZART1, was identified in humans and plants, but its precise functions remain to be determined. In this paper, we characterize the fission yeast homologue Mzt1, showing that it is essential for cell viability. Mzt1 is present in approximately equal stoichiometry with Alp4/GCP2 and localizes to all the MTOCs, including the SPB and interphase and equatorial MTOCs. Temperature-sensitive mzt1 mutants display varying degrees of compromised microtubule organization, exhibiting multiple defects during both interphase and mitosis. Mzt1 is required for γ-TuC recruitment, but not sufficient to localize to the SPB, which depends on γ-TuC integrity. Intriguingly, the core γ-TuC assembles in the absence of Mzt1. Mzt1 therefore plays a unique role within the γ-TuC components in attachment of this complex to the major MTOC site.

Dividing the spoils of growth and the cell cycle: The fission yeast as a model for the study of cytokinesis

Cytokinesis is the final stage of the cell cycle, and ensures completion of both genome segregation and organelle distribution to the daughter cells. Cytokinesis requires the cell to solve a spatial problem (to divide in the correct place, orthogonally to the plane of chromosome segregation) and a temporal problem (to coordinate cytokinesis with mitosis). Defects in the spatiotemporal control of cytokinesis may cause cell death, or increase the risk of tumor formation [Fujiwara et al., 2005 (Fujiwara T, Bandi M, Nitta M, Ivanova EV, Bronson RT, Pellman D. 2005. Cytokinesis failure generating tetraploids promotes tumorigenesis in p53-null cells. Nature 437:1043–1047); reviewed by Ganem et al., 2007 (Ganem NJ, Storchova Z, Pellman D. 2007. Tetraploidy, aneuploidy and cancer. Curr Opin Genet Dev 17:157–162.)]. Asymmetric cytokinesis, which permits the generation of two daughter cells that differ in their shape, size and properties, is important both during development, and for cellular homeostasis in multicellular organisms [reviewed by Li, 2007 (Li R. 2007. Cytokinesis in development and disease: variations on a common theme. Cell Mol Life Sci 64:3044–3058)]. The principal focus of this review will be the mechanisms of cytokinesis in the mitotic cycle of the yeast Schizosaccharomyces pombe. This simple model has contributed significantly to our understanding of how the cell cycle is regulated, and serves as an excellent model for studying aspects of cytokinesis. Here we will discuss the state of our knowledge of how the contractile ring is assembled and disassembled, how it contracts, and what we know of the regulatory mechanisms that control these events and assure their coordination with chromosome segregation.

Differential expression and cellular distribution of gamma-tubulin and betaIII-tubulin in medulloblastomas and human medulloblastoma cell lines

In previous studies, we have shown overexpression and ectopic subcellular distribution of gamma-tubulin and betaIII-tubulin in human glioblastomas and glioblastoma cell lines (Katsetos et al., 2006, J Neuropathol Exp Neurol 65:455-467; Katsetos et al., 2007, Neurochem Res 32:1387-1398). Here we determined the expression of gamma-tubulin in surgically excised medulloblastomas (n = 20) and in the human medulloblastoma cell lines D283 Med and DAOY. In clinical tissue samples, the immunohistochemical distribution of gamma-tubulin labeling was pervasive and inversely related to neuritogenesis. Overexpression of gamma-tubulin was widespread in poorly differentiated, proliferating tumor cells but was significantly diminished in quiescent differentiating tumor cells undergoing neuritogenesis, highlighted by betaIII-tubulin immunolabeling. By quantitative real-time PCR, gamma-tubulin transcripts for TUBG1, TUBG2, and TUBB3 genes were detected in both cell lines but expression was less prominent when compared with the human glioblastoma cell lines. Immunoblotting revealed comparable amounts of gamma-tubulin and betaIII-tubulin in different phases of cell cycle; however, a larger amount of gamma-tubulin was detected in D283 Med when compared with DAOY cells. Interphase D283 Med cells exhibited predominantly diffuse cytoplasmic gamma-tubulin localization, in addition to the expected centrosome-associated distribution. Robust betaIII-tubulin immunoreactivity was detected in mitotic spindles of DAOY cells. Our data indicate that overexpression of gamma-tubulin may be linked to phenotypic dedifferentiation (anaplasia) and tumor progression in medulloblastomas and may potentially serve as a promising tumor marker.

The environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin disturbs the establishment and maintenance of cell polarity in preimplantation rat embryos

Maternal exposure to the environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces a variety of defects in compaction-stage embryos, including monopolar spindle formation, errors in chromosome segregation, and fragmentation resulting from aberrant cytokinesis. In this study, we investigated the possibility that a failure in centrosome duplication, separation, or positioning within blastomeres might underlie the observed effects of TCDD on early embryos. The subcellular localization of the centrosomal marker TUBG1 was analyzed in preimplantation embryos collected from female rats exposed to either chronic (50 ng kg(-1) wk(-1) for 3 wk) or acute (50 ng/kg or 1 microg/kg at proestrus) doses of TCDD. In treated embryos, interphase TUBG1 foci were more abundant and cortically displaced when compared to those in controls. At prophase, some blastomeres exhibited a single large perinuclear TUBG1 aggregate, suggesting a failure in centrosome duplication or separation. Furthermore, the presence of monopolar spindles at metaphase was confirmed by the localization of TUBG1 to the single spindle pole. Therefore, the misregulation of centrosome number and localization, as indicated by TUBG1 staining, may contribute to errors in chromosome segregation and cytokinesis in embryos following maternal TCDD exposure.

New and old reagents for fluorescent protein tagging of microtubules in fission yeast; experimental and critical evaluation

The green fluorescent protein (GFP) has become a mainstay of in vivo imaging in many experimental systems. In this chapter, we first discuss and evaluate reagents currently available to image GFP-labeled microtubules in the fission yeast Schizosaccharomyces pombe, with particular reference to time-lapse applications. We then describe recent progress in the development of robust monomeric and tandem dimer red fluorescent proteins (RFPs), including mCherry, TagRFP-T, mOrange2, mKate, and tdTomato, and we present data assessing their suitability as tags in S. pombe. As part of this analysis, we introduce new PCR tagging cassettes for several RFPs, new pDUAL-based plasmids for RFP-tagging, and new RFP-tubulin strains. These reagents should improve and extend the study of microtubules and microtubule-associated proteins in S. pombe.

Distinct Dgrip84 isoforms correlate with distinct gamma-tubulins in Drosophila

Gamma-tubulin is an indispensable component of the animal centrosome and is required for proper microtubule organization. Within the cell, gamma-tubulin exists in a multiprotein complex containing between two (some yeasts) and six or more (metazoa) additional highly conserved proteins named gamma ring proteins (Grips) or gamma complex proteins (GCPs). gamma-Tubulin containing complexes isolated from Xenopus eggs or Drosophila embryos appear ring-shaped and have therefore been named the gamma-tubulin ring complex (gammaTuRC). Curiously, many organisms (including humans) have two distinct gamma-tubulin genes. In Drosophila, where the two gamma-tubulin isotypes have been studied most extensively, the gamma-tubulin genes are developmentally regulated: the "maternal" gamma-tubulin isotype (named gammaTub37CD according to its location on the genetic map) is expressed in the ovary and is deposited in the egg, where it is thought to orchestrate the meiotic and early embryonic cleavages. The second gamma-tubulin isotype (gammaTub23C) is ubiquitously expressed and persists in most of the cells of the adult fly. In those rare cases where both gamma-tubulins coexist in the same cell, they show distinct subcellular distributions and cell-cycle-dependent changes: gammaTub37CD mainly localizes to the centrosome, where its levels vary only slightly with the cell cycle. In contrast, the level of gammaTub23C at the centrosome increases at the beginning of mitosis, and gammaTub23C also associates with spindle pole microtubules. Here, we show that gammaTub23C forms discrete complexes that closely resemble the complexes formed by gammaTub37CD. Surprisingly, however, gammaTub23C associates with a distinct, longer splice variant of Dgrip84. This may reflect a role for Dgrip84 in regulating the activity and/or the location of the gamma-tubulin complexes formed with gammaTub37CD and gammaTub23C.

Modulation of Alp4 function in Schizosaccharomyces pombe induces novel phenotypes that imply distinct functions for nuclear and cytoplasmic gamma-tubulin complexes

The gamma-tubulin complex acts as a nucleation unit for microtubule assembly. It remains unknown, however, how spatial and temporal regulation of the complex activity affects microtubule-mediated cellular processes. Alp4 is one of the essential components of the S. pombe gamma-tubulin complex. We show here that overproduction of a carboxy-terminal form of Alp4 (Alp4C) and its derivatives tagged to a nuclear localization signal or to a nuclear export signal affect localization of gamma-tubulin complexes and induces novel phenotypes that reflect distinct functions of nuclear and cytoplasmic gamma-tubulin complexes. Nuclear Alp4C induces a Wee1-dependent G2 delay, reduces the levels of the gamma-tubulin complex at the spindle pole body, and results in defects in mitotic progression including spindle assembly, cytoplasmic microtubule disassembly, and chromosome segregation. In contrast, cytoplasmic Alp4C induces oscillatory nuclear movement and affects levels of cell polarity markers, Bud6 and Tip1, at the cell ends. These results demonstrate that regulation of nuclear gamma-tubulin complex activity is essential for cell cycle progression through the G2/M boundary and M phase, whereas regulation of cytoplasmic gamma-tubulin complex activity is important for nuclear positioning and cell polarity control during interphase.

Isolation of mutations that bypass the requirement of the septation initiation network for septum formation and conidiation in Aspergillus nidulans

The kinase cascade of the septation initiation network (SIN), first revealed in fission yeast, activates the contraction of the actomyosin ring, and plays an essential role in fungal septation. Mob1p, an evolutionarily conserved SIN protein, is associated with the most downstream kinase of this cascade in fission yeast. In this study, the mobA gene encoding a homologous protein was isolated from the filamentous fungus Aspergillus nidulans, whose mycelium is made of multinucleate cells. The MOBA protein was required for septation and conidiation, but was not essential for hyphal extension and colony formation. To identify genes that act antagonistically against the SIN, UV mutagenesis was carried out to isolate suppressor (smo) mutations that restored conidiation when MOBA was not expressed. Microscopic examination indicated that the restored conidiation was concomitant with restored septation in the absence of the MOBA protein. Eight recessive smo mutations in five complementation groups also bypassed the requirement of the SIN kinases SEPH and SIDB for septum formation and conidiation. However, none of these smo mutations affected the localization of MOBA. Among smo mutations, smoA and smoB mutations caused reduced hyphal growth and colony formation. They also rendered hypersensitivity to low doses of the microtubule-depolymerizing agent benomyl for conidiation. Therefore, in A. nidulans, proteins encoded by the smo genes likely have an antagonistic interaction against the SIN pathway to regulate septation and conidiation.

The Drosophila gamma-tubulin small complex subunit Dgrip84 is required for structural and functional integrity of the spindle apparatus

Gamma-tubulin, a protein critical for microtubule assembly, functions within multiprotein complexes. However, little is known about the respective role of gamma-tubulin partners in metazoans. For the first time in a multicellular organism, we have investigated the function of Dgrip84, the Drosophila orthologue of the Saccharomyces cerevisiae gamma-tubulin-associated protein Spc97p. Mutant analysis shows that Dgrip84 is essential for viability. Its depletion promotes a moderate increase in the mitotic index, correlated with the appearance of monopolar or unpolarized spindles, impairment of centrosome maturation, and increase of polyploid nuclei. This in vivo study is strengthened by an RNA interference approach in cultured S2 cells. Electron microscopy analysis suggests that monopolar spindles might result from a failure of centrosome separation and an unusual microtubule assembly pathway via centriolar triplets. Moreover, we point to an involvement of Dgrip84 in the spindle checkpoint regulation and in the maintenance of interphase microtubule dynamics. Dgrip84 also seems essential for male meiosis, ensuring spindle bipolarity and correct completion of cytokinesis. These data sustain that Dgrip84 is required in some aspects of microtubule dynamics and organization both in interphase and mitosis. The nature of a minimal gamma-tubulin complex necessary for proper microtubule organization in the metazoans is discussed.

Cytoplasmic dynein's mitotic spindle pole localization requires a functional anaphase-promoting complex, gamma-tubulin, and NUDF/LIS1 in Aspergillus nidulans

In Aspergillus nidulans, cytoplasmic dynein and NUDF/LIS1 are found at the spindle poles during mitosis, but they seem to be targeted to this location via different mechanisms. The spindle pole localization of cytoplasmic dynein requires the function of the anaphase-promoting complex (APC), whereas that of NUDF does not. Moreover, although NUDF's localization to the spindle poles does not require a fully functional dynein motor, the function of NUDF is important for cytoplasmic dynein's targeting to the spindle poles. Interestingly, a gamma-tubulin mutation, mipAR63, nearly eliminates the localization of cytoplasmic dynein to the spindle poles, but it has no apparent effect on NUDF's spindle pole localization. Live cell analysis of the mipAR63 mutant revealed a defect in chromosome separation accompanied by unscheduled spindle elongation before the completion of anaphase A, suggesting that gamma-tubulin may recruit regulatory proteins to the spindle poles for mitotic progression. In A. nidulans, dynein is not apparently required for mitotic progression. In the presence of a low amount of benomyl, a microtubule-depolymerizing agent, however, a dynein mutant diploid strain exhibits a more pronounced chromosome loss phenotype than the control, indicating that cytoplasmic dynein plays a role in chromosome segregation.

Mto2p, a novel fission yeast protein required for cytoplasmic microtubule organization and anchoring of the cytokinetic actin ring

Microtubules regulate diverse cellular processes, including chromosome segregation, nuclear positioning, and cytokinesis. In many organisms, microtubule nucleation requires gamma-tubulin and associated proteins present at specific microtubule organizing centers (MTOCs). In fission yeast, interphase cytoplasmic microtubules originate from poorly characterized interphase MTOCs and spindle pole body (SPB), and during late anaphase from the equatorial MTOC (EMTOC). It has been previously shown that Mto1p (Mbo1p/Mod20p) function is important for the organization/nucleation of all cytoplasmic microtubules. Here, we show that Mto2p, a novel protein, interacts with Mto1p and is important for establishing a normal interphase cytoplasmic microtubule array. In addition, mto2Delta cells fail to establish a stable EMTOC and localize gamma-tubulin complex members to this medial structure. As predicted from these functions, Mto2p localizes to microtubules, the SPB, and the EMTOC in an Mto1p-dependent manner. mto2Delta cells fail to anchor the cytokinetic actin ring in the medial region of the cell and under conditions that mildly perturb actin structures, these rings unravel in mto2Delta cells. Our results suggest that the Mto2p and the EMTOC are critical for anchoring the cytokinetic actin ring to the medial region of the cell and for proper coordination of mitosis with cytokinesis.

Effects of {gamma}-tubulin complex proteins on microtubule nucleation and catastrophe in fission yeast

Although gamma-tubulin complexes (gamma-TuCs) are known as microtubule (MT) nucleators, their function in vivo is still poorly defined. Mto1p (also known as mbo1p or mod20p) is a gamma-TuC-associated protein that recruits gamma-TuCs specifically to cytoplasmic MT organizing centers (MTOCs) and interphase MTs. Here, we investigated gamma-TuC function by analyzing MT behavior in mto1Delta and alp4 (GCP2 homologue) mutants. These cells have free, extra-long interphase MTs that exhibit abnormal behaviors such as cycles of growth and breakage, MT sliding, treadmilling, and hyperstability. The plus ends of interphase and spindle MTs grow continuously, exhibiting catastrophe defects that are dependent on the CLIP170 tip1p. The minus ends of interphase MTs exhibit shrinkage and pauses. As mto1Delta mutants lack cytoplasmic MTOCs, cytoplasmic MTs arise from spindle or other intranuclear MTs that exit the nucleus. Our findings show that mto1p and gamma-TuCs affect multiple properties of MTs including nucleation, nuclear attachment, plus-end catastrophe, and minus-end shrinkage.

Localization of gamma-tubulin in mouse eggs during meiotic maturation, fertilization, and early embryonic development

Gamma-tubulin, a member of the tubulin superfamily, is a peri-centriolar component which is considered to be essential for microtubule nucleation. The dynamics of gamma-tubulin during mouse oocyte meiotic maturation, fertilization, and early cleavage as well as the co-localization of gamma-tubulin and alpha-tubulin during the formation of the meiotic I spindle were studied by confocal microscopy. We found that gamma-tubulin was evenly distributed in the germinal vesicle (GV) stage oocyte. After germinal vesicle breakdown (GVBD) gamma-tubulin dots were localized in both the cytoplasm and the vicinity of the condensed chromosomes, and aligned at both poles of the meiotic spindle at prometaphase I and metaphase I. At anaphase I and telophase I, gamma-tubulin was detected between the separating chromosomes, while it was absent in the midbody. At the MII stage, gamma-tubulin was again accumulated at the spindle poles. Alpha-tubulin had a similar distribution pattern as gamma-tubulin in the cytoplasm and radiated from gamma-tubulin foci close to the chromosomes during the meiotic spindle formation. After fertilization, gamma-tubulin was translocated from spindle poles to the area between separating chromatids and distributed around the pronuclei. It aggregated into some dots during the interphase, but was distributed on the mitotic spindle poles in early embryos. Our results suggest that gamma-tubulin is essential for microtubule nucleation and spindle formation during mouse oocyte meiosis, fertilization, and early embryo cleavage.

Events at the end of mitosis in the budding and fission yeasts

The mitotic exit network (MEN) and the septation initiation network (SIN) control events at the end of mitosis in S. cerevisiae and S. pombe, respectively. SIN initiates contraction of the actin ring and synthesis of the division septum, thereby bringing about cytokinesis. The MEN is also required for cytokinesis, but its main role is to control inactivation of mitotic cyclin-dependent kinases (CDKs) at the end of mitosis, and thereby regulate mitotic exit. Each revolves around a Ras-family GTPase and involves several protein kinases, and SIN and MEN proteins are localised to the spindle pole body. In S. cerevisiae, a second network, known as FEAR, cooperates with the MEN to bring about mitotic exit, and a third, AMEN, contributes to switching the MEN off. Some of the central components of the FEAR, SIN and MEN have been conserved through evolution, which suggests that aspects of their function in controlling events at the end of mitosis might be conserved in higher eukaryotes.

Rsp1p, a J Domain Protein Required for Disassembly and Assembly of Microtubule Organizing Centers during the Fission Yeast Cell Cycle

Regulation of microtubule organizing centers (MTOCs) orchestrates the reorganization of the microtubule (MT) cytoskeleton. In the fission yeast Schizosaccharomyces pombe, an equatorial MTOC (eMTOC) at the cell division site disassembles after cytokinesis, and multiple interphase MTOCs (iMTOCs) appear on the nucleus. Here, we show that, upon eMTOC disassembly, small satellites carrying MTOC components such as the gamma-tubulin complex travel in both directions along interphase MTs. We identify rsp1p, an MTOC protein required for eMTOC disassembly. In rsp1 loss-of-function mutants, the eMTOC persists and organizes an abnormal microtubule aster, while iMTOCs and satellites are greatly reduced. Conversely, rsp1p overexpression inhibits eMTOC formation. Rsp1p is a J domain protein that interacts with an hsp70. Thus, our findings suggest a model in which rsp1p is part of a chaperone-based mechanism that disassembles the eMTOC into satellites, contributing to the dynamic redistribution of MTOC components for organization of interphase microtubules.

Identification and characterization of two novel proteins affecting fission yeast gamma-tubulin complex function

The gamma-tubulin complex, via its ability to organize microtubules, is critical for accurate chromosome segregation and cytokinesis in the fission yeast, Schizosaccharomyces pombe. To better understand its roles, we have purified the S. pombe gamma-tubulin complex. Mass spectrometric analyses of the purified complex revealed known components and identified two novel proteins (i.e., Mbo1p and Gfh1p) with homology to gamma-tubulin-associated proteins from other organisms. We show that both Mbo1p and Gfh1p localize to microtubule organizing centers. Although cells deleted for either mbo1(+) or gfh1(+) are viable, they exhibit a number of defects associated with altered microtubule function such as defects in cell polarity, nuclear positioning, spindle orientation, and cleavage site specification. In addition, mbo1Delta and gfh1Delta cells exhibit defects in astral microtubule formation and anchoring, suggesting that these proteins have specific roles in astral microtubule function. This study expands the known roles of gamma-tubulin complex components in organizing different types of microtubule structures in S. pombe.

gamma-tubulin plays an essential role in the coordination of mitotic events

Recent data from multiple organisms indicate that gamma-tubulin has essential, but incompletely defined, functions in addition to nucleating microtubule assembly. To investigate these functions, we examined the phenotype of mipAD159, a cold-sensitive allele of the gamma-tubulin gene of Aspergillus nidulans. Immunofluorescence microscopy of synchronized material revealed that at a restrictive temperature mipAD159 does not inhibit mitotic spindle formation. Anaphase A was inhibited in many nuclei, however, and after a slight delay in mitosis (approximately 6% of the cell cycle period), most nuclei reentered interphase without dividing. In vivo observations of chromosomes at a restrictive temperature revealed that mipAD159 caused a failure of the coordination of late mitotic events (anaphase A, anaphase B, and chromosomal disjunction) and nuclei reentered interphase quickly even though mitosis was not completed successfully. Time-lapse microscopy also revealed that transient mitotic spindle abnormalities, in particular bent spindles, were more prevalent in mipAD159 strains than in controls. In experiments in which microtubules were depolymerized with benomyl, mipAD159 nuclei exited mitosis significantly more quickly (as judged by chromosomal condensation) than nuclei in a control strain. These data reveal that gamma-tubulin has an essential role in the coordination of late mitotic events, and a microtubule-independent function in mitotic checkpoint control.

Fission yeast cdc31p is a component of the half-bridge and controls SPB duplication

The fission yeast spindle pole body (SPB) is a nucleus-associated organelle that duplicates once each cell cycle during interphase. Duplicated SPBs serve as the poles of an intranuclear mitotic spindle after their insertion into the nuclear envelope in mitosis (Ding et al., Mol. Biol. Cell 8, 1461-1479). Here, we report the identification and characterization of Schizosaccharomyces pombe cdc31p, a member of the conserved calcium-binding centrin/CDC31 family. Immunofluorescence and immunoelectron microscopy show that cdc31p is a SPB component localized at the half-bridge structure of the SPB. cdc31 is an essential gene and Deltacdc31 cells and cdc31 conditional mutant cells arrest in mitosis with a monopolar mitotic spindle organized from a single SPB. EM analysis demonstrates that mutant cdc31 cells fail to duplicate the SPB. In addition, cdc31p exhibits genetic interactions with the SPB component sad1p and is required for sad1p localization. Finally, cdc31 mutant can undergo single or multiple rounds of septation before the exit from mitosis, suggesting that cdc31p activity or SPB duplication may be required for the proper coordination between the exit from mitosis and the initiation of septation.

Microtubule nucleation

Microtubule nucleation is the process in which several tubulin molecules interact to form a microtubule seed. Microtubule nucleation occurs spontaneously in purified tubulin solutions, and molecular intermediates between tubulin dimers and microtubules have been identified. Microtubule nucleation is enhanced in tubulin solutions by the addition of gamma-tubulin or various gamma-tubulin complexes. In vivo, microtubule assembly is usually seeded by gamma-tubulin ring complexes. Recent studies suggest, however, that microtubule nucleation can occur in the absence of gamma-tubulin, and that gamma-tubulin may have other cell functions apart from being a major component of the gamma-tubulin ring complex.

Spindle-kinetochore attachment requires the combined action of Kin I-like Klp5/6 and Alp14/Dis1-MAPs in fission yeast

Fission yeast Klp5 and Klp6 belong to the microtubule-destabilizing Kin I family. In klp5 mutants, spindle checkpoint proteins Mad2 and Bub1 are recruited to mitotic kinetochores for a prolonged duration, indicating that these kinetochores are unattached. Further analysis shows that there are kinetochores to which only Bub1, but not Mad2, localizes. These kinetochores are likely to have been captured, yet lack tension. Thus Klp5 and Klp6 play a role in a spindle- kinetochore interaction at dual steps, capture and generation of tension. The TOG/XMAP215 family, Alp14 and Dis1 are known to stabilize microtubules and be required for the bivalent attachment of the kinetochore to the spindle. Despite apparent opposing activities towards microtubule stability, Klp5/Klp6 and Alp14/Dis1 share an essential function, as either dis1klp or alp14klp mutants are synthetically lethal, like alp14dis1. Defective phenotypes are similar to each other, characteristic of attachment defects and chromosome mis-segregation. Furthermore Alp14 is of significance for kinetochore localization of Klp5. We propose that Klp5/Klp6 and Alp14/Dis1 play a collaborative role in bipolar spindle formation during prometaphase through producing spindle dynamism.

A fourth component of the fission yeast gamma-tubulin complex, Alp16, is required for cytoplasmic microtubule integrity and becomes indispensable when gamma-tubulin function is compromised

gamma-Tubulin functions as a multiprotein complex, called the gamma-tubulin complex (gamma-TuC), and composes the microtubule organizing center (MTOC). Fission yeast Alp4 and Alp6 are homologues of two conserved gamma-TuC proteins, hGCP2 and hGCP3, respectively. We isolated a novel gene, alp16(+), as a multicopy suppressor of temperature-sensitive alp6-719 mutants. alp16(+) encodes a 759-amino-acid protein with two conserved regions found in all other members of gamma-TuC components. In addition, Alp16 contains an additional motif, which shows homology to hGCP6/Xgrip210. Gene disruption shows that alp16(+) is not essential for cell viability. However, alp16 deletion displays abnormally long cytoplasmic microtubules, which curve around the cell tip. Furthermore, alp16-deleted mutants are hypersensitive to microtubule-depolymerizing drugs and synthetically lethal with either temperature-sensitive alp4-225, alp4-1891, or alp6-719 mutants. Overproduction of Alp16 is lethal, with defective phenotypes very similar to loss of Alp4 or Alp6. Alp16 localizes to the spindle pole body throughout the cell cycle and to the equatorial MTOC at postanaphase. Alp16 coimmunoprecipitates with gamma-tubulin and cosediments with the gamma-TuC in a large complex (>20 S). Alp16 is, however, not required for the formation of this large complex. We discuss evolutional conservation and divergence of structure and function of the gamma-TuC between yeast and higher eukaryotes.